What does that tell you? Well, it tells you many important things. It tells you that... that what caused the cessation of mitotic activity in the male cells in the mixed culture could not be a virus unless you invoke the exceedingly unlikely event and that is that some virus could distinguish between male and female cells and because we know that that's highly improbable – I won't say impossible, but I'll say very highly improbable – you can exclude that as an explanation. Furthermore, you... on the same basis, you can exclude technical error, which if it occurred could not possibly affect only one sex cells and not the other. You could also dismiss the theory that it had something to do with media composition for the same reason. Why would media distinguish male from female cells? Certainly an extreme possibility but highly improbable.

And that was pretty much the key experiment of the several that we did in those days to prove that our cells were normal. Of course, Paul's findings in respect to the side of genetics indicated clearly they were normal. But this male-female experiment which we at first because of the way I just explained it, we called this experiment the 'dirty old man experiment' for obvious reasons. I should say that we also did the 'dirty old lady' experiment with the same result. So we were pretty convinced that this was not an artefact of culture, that the cells' failure to divide after roughly 50 population doublings was due to some kind of internal cellular mechanism and not some external artefact like virus, media composition, culture failure, phases of the moon, anything else that you could invoke. And so this, of course, flew in the face of the dogma that said that all cultures have the potential to be immortal.

Leonard Hayflick (b. 1928), the recipient of several research prizes and awards, including the 1991 Sandoz Prize for Gerontological Research, is known for his research in cell biology, virus vaccine development, and mycoplasmology. He also has studied the ageing process for more than thirty years. Hayflick is known for discovering that human cells divide for a limited number of times in vitro (refuting the contention by Alexis Carrel that normal body cells are immortal), which is known as the Hayflick limit, as well as developing the first normal human diploid cell strains for studies on human ageing and for research use throughout the world. He also made the first oral polio vaccine produced in a continuously propogated cell strain - work which contributed to significant virus vaccine development.